EiR: Doping cubic boron nitride, an extreme material for power electronics and radiation detection

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This project focuses on cubic boron nitride for extreme applications. Boron nitride is member of the family of semiconductors known as III-nitrides, which consist of nitrogen and a Group III element such as aluminum, gallium, and indium. This family of materials has realized the solid-state lighting industry and revolutionized microwave communications. Cubic boron nitride is an ultra-wide bandgap material, transparent to visible light and absorbing only in the deep ultraviolet. It could have a transformative impact on power electronics for electric vehicles, smart grids, and space technologies. Its exceptional hardness, high thermal conductivity and corrosion resistance could also allow this material to replace diamond in harsh environments. Studies of boron nitride will also contribute to fundamental knowledge of III-nitride semiconductors. The project is a collaboration between Howard and Morgan State University, two Historically Black Colleges (HBCUs). The project enables Morgan and Howard students to participate in a cutting edge technology with commercial implications. The project explicitly engages both graduate students and undergraduates in "hands on" research activities. The project will fund senior design projects at Morgan and Howard, where students will translate physical electronics discoveries into real systems. The project will encourage greater minority participation in graduate research programs at HBCUs. To realize the full potential of cubic boron nitride (c-BN), it is desirable to have both n-type and p-type conduction enabling bipolar devices as well as Complementary Metal Oxide Semiconductor (CMOS) logic. It has been demonstrated that shallow n-type (silicon doping), and p-type (beryllium doping) are possible, making c-BN a unique outlier among ultrawide bandgap materials (UWBGs). In this work, we investigate doping and compensation strategies to evaluate the promise of c-BN. In addition we measure physical properties of this material such as mobility and breakdown field as a function of defect morphology (updating historical data as necessary). The work involves both small (1 mm) commercial single-crystal c-BN samples, as well as large area thin films grown at Morgan State. All measurements will be interpreted by comparing with first-principles density functional theory (DFT) calculations that provide a true atomic physics-based understanding. One of the challenges of the work will be to produce polytypic pure material. In order to accomplish this during chemical vapor deposition (CVD), the less stable hexagonal boron nitride (h-BN) polymorph is etched or sputtered away allowing the production of the cubic phase. We will test 3 methods to do this: i) etching by H2, ii) use of fluorine-based precursors and iii) ion-beam assistance, on single-crystal HPHT diamond substrates. Surface preparation and post-annealing will be guided by DFT calculations. Compensation and doping by intrinsic point defects (e.g. vacancies) are likely to play a strong role in this UWBG material, and DFT will be a critical tool to understanding the origin of the doping, and electrical transport. Diodes and metal-semiconductor-metal structures will be used as test devices.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.